The present invention relates to a friction engagement apparatus capable of preferable control of a transmission force through friction engagement, and to a friction engagement control method therefor.
Friction engagement apparatuses utilizing a friction force for transmission are known and used in various kinds of equipment. Taking a vehicle as an example, a clutch mounted inside an automatic transmission is one example of a friction engagement apparatus. A friction brake, mounted to a wheel, can also be viewed as a friction engagement apparatus.
Conventionally, desirable control of a transmission force of a friction engagement apparatus requires a mechanical hydraulic control mechanism for varying a pushing force (pressing force) which presses the friction surface.
However, if such a transmission force of a friction engagement apparatus could be controlled by any means other than mechanical control, various advantages, including simplification of an apparatus, would be possible.
As a reference, Japanese Patent Application Laid-open No. Hei 8-135688 discloses a technique utilizing electrical viscous liquid. The viscosity of an electrical viscous liquid varies when different voltages are applied to the liquid, so that a transmission force can be varied through voltage control. However, as a transmission force is varied solely through variation of the viscosity, only a small transmission force can be generated. Moreover, as the unit is filled with liquid, large agitation resistance is generated.
As another reference, Japanese Patent Application Laid-open No. Hei 10-26150 discloses an electrically controlled motion transmission apparatus in which a drive body and a driven body are arranged opposing each other and having a space between them, and wherein an electric conductive fiber is placed in the space. Voltage application to the space between the bodies causes the fiber to stand due to electrostatic attraction, generating a shear stress between the opposed surfaces of the respective bodies. A transmission force can be obtained by means of the shear force and controlled by controlling the voltage applied.
Similar techniques are also disclosed in Japanese Patent Application Laid-open Nos. Hei 9-53663 and Hei 10-96435.
The above mentioned techniques, however, can generate only a small transmission force in a limited range because they basically obtain a transmission force from electrostatic attraction of fiber, and only minute electrostatic attraction can be obtained by applying a voltage. An arrangement having electrical viscous fluid filling the space between the drive and driven bodies likewise can generate only a small transmission force.
As described above, conventional transmission force control is achieved mainly using a mechanical hydraulic control mechanism. Although techniques for controlling liquid viscosity or electrostatic attraction through voltage application are available, because mechanical friction engagement is interrupted, only a small transmission force is obtainable.
Against this background, the object of the present invention is to provide a preferable friction engagement apparatus capable of providing a desired transmission force.
In order to achieve the above object, the present invention utilizes the fact that a friction coefficient of a friction surface of a friction engagement apparatus varies upon impartation of an electric field thereto, and controls a transmission force utilizing this phenomenon.
A friction engagement apparatus of the present invention comprises a plurality of friction engagement members for mutual friction engagement; a pushing force generation device for generating a pushing force bringing the plurality of friction engagement members into friction engagement; and an electric field imparting device for imparting an electric field to a space between the plurality of friction engagement members to change friction coefficients of friction surfaces.
In a friction engagement apparatus of the present invention, a plurality of friction engagement members are in direct friction engagement by the action of a pushing force. An electric field imparting device imparts an electric field to spaces between a plurality of friction engagement members. The imparting of an electric field causes a friction coefficient of a friction surface between the friction engagement members to vary. A transmission force follows a pushing force and a friction coefficient of the friction surface. Therefore, it is possible to control a transmission force of the friction engagement apparatus by imparting an electric field.
Preferably, a control device for controlling the strength of an electric field imparted by the electric field imparting device is provided. A friction coefficient of a friction surface increases or decreases according to the strength of an electric field, as a result of which a transmission force varies.
Also preferably, the friction surfaces of the plurality of friction engagement members are lubricated by a lubrication agent, and the lubrication agent may preferably contain a polarized additive. It is assumed that the polarized additive is absorbed in a friction surface under influence of an electric field, and that a friction coefficient of the friction surface vary due to the absorption of the additive.
Further preferably, at least one of the friction engagement members is provided with an insulative friction member, so that an input side and an output side are in mutual contact via the friction member. This makes it possible to readily impart a desired electric field. Power consumption is advantageously reduced.
A friction engagement apparatus may be a clutch or brake device, for example.
A friction engagement apparatus may include a pushing force control device for increasing or decreasing a pushing force.
According to a friction engagement control method of the present invention, a transmission force between a plurality of friction engagement members is controlled by imparting an electric field to spaces between the plurality of friction engagement members which are in mutual friction contact.